Method of producing a corrosion resistant alloy of cu-ni by liquid phase sintering



Unite iii 3,441,409 METHOD OF PRODUCING A CORROSION RE- SISTANT ALLOY OF Cu-Ni BY LIQUID PHASE SINTERING Malcolm F. Burr, Bethlehem, Cnn., assignor to Chase Brass and Copper Co., Inc., Waterbury, C0nn., a corporation of Connecticut No Drawing. Filed Jan. 26, 1967, Ser. No. 611,843 Int. Cl. C22c 1/04, 19/00 US. Cl. 752(l8 12 Claims ABSTRACT OF THE DISCLOSURE Corrosion resistant alloys of copper and nickel, such as Monel metal and the cupro-nickel alloys, are well known in the art. Such alloys are very useful as structural materials and as protective or cladding materials in many corrosive environments which would attack ordinary steel and unalloyed copper. Such alloys are ordinarily produced by mixing the required quantities of molten copper with molten nickel to form a homogeneous alloy of the desired composition. The resulting molten alloy is then cast into billets or slabs which are machined or rolled or otherwise fabricated to produce the desired end product. However these alloys, and particularly the alloys which contain a relatively high proportion of nickel, are ditficult to fabricate and especially to employ in the form of thin protective coatings or layers on substrates of structural materials such as steel. Moreover, the conventional processes by which these essentially homogeneous copper-nickel alloys are produced and fabricated into useful end products are complicated, time consuming and relatively costly.

I have now developed a new corrosion resistant copper'nickel alloy that is readily fabricated into useful end products directly from its metallic copper and metallic nickel constituents without the usual time consuming and costly steps of prior procedures. The alloy of my invention contains from about 20 to 50 percent, and preferably about percent, by weight copper and from about to 80 percent, and preferably about percent, by weight nickel, the alloy comprising a coherent metal body composed of mutually adhering particles of a nonhomogeneous copper-nickel alloy. That is to say, the composition of the alloy at the surface of each of said mutually adhering particles is richer in copper than in nickel and at the center thereof is richer in nickel than in copper, the surface layer of each particle advantageously containing not more than percent by weight copper and the center of each particle advantageously containing about 100 percent by weight nickel.

The corrosion resistant nonhomogeneous alloy of my invention is produced by forming a mixture of from about 20 percent to 50 percent by weight, and preferably about 35 percent by weight, of finely divided copper powder and from about 50 percent to percent by weight, and preferably about 65 percent by weight, of nickel powder, the average particle size of the copper States Patent 0 ice powder being no greater than and advantageously being less than the average particle size of the nickel powder. The mixture of metal powder is formed into an article of the desired shape by any of the techniques hereinafter described, and the shaped mixture of powder is heated in a reducing atmosphere to a temperature above the melting point of copper and below the melting point of nickel for a time sufiicient to insure that the copper will melt and will alloy with the unmelted particles of nickel powder. The copper diffusing into the particles of nickel transforms each particle in a nonhomogeneous copper-nickel alloy containing, advantageously, about 70 percent copper at the surface of each particle and about percent nickel at the center of each particle, the particles being fused together at their contacting surfaces to form a coherent body of mutually adhering particles of said nonhomogeneous alloy.

In the practice of my invention finely divided copper powder and nickel powder are mixed together in the proportions specified herein so that when the mixture is heated in a reducing atmosphere to a temperature above the melting point of copper and below the melting point of nickel the copper will melt and completely dissolve or diffuse into the unmelted particles of nickel. Specifical- 1y I have found that the total amount of copper in the initial mixture should not exceed about 50 percent by weight in order to minimize the possibility of incomplete solution of the molten copper in the nickel particles, and further that it should contain not less than 20 percent by weight of copper in order to obtain a corrosion resistant alloy of the desired composition. I have further found that the particle size of the copper powder should be fine, and may be substantially smaller than, the particlcs of nickel powder in the initial mixture. That is to say, the nickel and copper powder can have the same screen analysis; however, the copper powder is preferably appreciably finer than the nickel powder in order to obtain a more uniform diffusion of the molten copper into the nickel particles. Thus, in the preferred practice of my invention the initial mixture contains about 35 percent by weight copper and about 65 percent by weight nickel, while the particles of nickel powder range from about 1 to 200 microns in size, and preferably in the order of 20 to 100 microns in size, and the particles of copper powder preferably are smaller than about 44 microns in size.

The copper and nickel powders are thoroughly blended together and the mixture of metal powder is then formed by any appropriate technique into an article of the desired shape. For example, a quantity of the mixture of metal powder can be placed in a die cavity where the mixture is compressed to form a metal powder compact in accordance with the techniques of powder metallurgy. The metal powder compact is then heated in a reducing atmosphere in accordance with my invention to produce the corrosion resistant nonhomogeneous alloy herein de scribed. Alternatively, the mixture of metal powder can be spread in a layer of the desired thickness on a supporting surface such as the moving metal belt shown in US. Patent 2,935,402, the layer of metal powder being compacted and then heated in accordance with my invention to produce a continuous sheet of nonhomogeneous copper-nickel alloy. In still another embodiment of the process of my invention, the mixture of metal powder is admixed with a volatile binder to form a plastic mass which is then extruded or otherwise formed into an extruded article of the desired shape as described in US. Patents 2,953,943 and 2,792,302, the extruded shape then being heated in accordance with my invention to produce nonhomogeneous copper-nickel alloy extrusions of any desired cross section. The binder employed to bond the metal particles together may be paraffin, heat fugitive thermoplastic resins, distillable hydrocarbon waxes or, in a particularly advantageous embodiment of the practice of my invention, the binder may comprise a solution of a a heat fugitive resin in a volatile organic solvent. In the latter embodiment, the mixture of metal powder and resinous solution is advantageously spread on a supporting surface in a layer of the desired thickness. The layer of metal powder and resinous solution is first heated to a temperature above the boiling point of the volatile solvent and below the boiling point or distillation range of the heat fugitive resin to obtain a self-supporting sheet or strip of the metal powder bonded together by the heat fugitive resin. The self-supporting strip or sheet of metal powder is then heated in accordance with my invention to drive off the heat fugitive resin and to form the nonhomogeneous copper-nickel alloy as herein more fully described.

The shaped mixture of copper and nickel powder is heated in a reducing atmosphere at a temperature in eX- cess of the melting point of copper and below the melting point of nickel for a time sufficient to insure that substantially all of the copper will melt and diffuse into the particles of nickel. The resulting nonhomogeneous alloy structure comprises a porous mass of particles of a nonhomogeneous copper-nickel alloy that are firmly bonded together into a coherent body by the fusion or welding of the individual particles at their contacting surfaces. The composition of the alloy as a whole is the same as that of the initial mixture of copper and nickel powder. However, the composition of each particle of the alloy that makes up the alloy body varies widely from the surface of each particle to the center thereof. That is to say, the composition of each particle of the alloy body is appreciably richer in copper than in nickel at the surface of the particle and is appreciably richer in nickel than in copper at the center of the particle. Thus, in a typical case the surface layer of the particle contains about 70 percent by weight copper and the center of the particle contains about 100 percent nickel.

The heating operation is carried out in a reducing atmosphere to avoid oxidation of the molten copper and of any organic binder that may be present in the mixture, and I presently prefer to use a reducing atmosphere containing from about 6 to 100 percent hydrogen, although other conventional reducing atmospheres known in the art can be employed. As noted, the temperature at which the metal powder shape is heated is above the melting point of copper and below the melting point of a 50%- 50% Cu-Ni alloy, i.e., 1275 C., and preferably is about 1200 C. The length of time required to effect the substantially complete solution of molten copper in the unmelted particles of nickel depends on the size, shape and particularly the thickness of the shaped mixture of powders. In a typical case where the thickness of the metal powder shape is not greater than about Mr inch, substantially complete solution of the molten copper in the nickel particles is achieved within about one minute at 1100 C.

The nonhomogeneous alloy body obtained as a result of the heating step possesses the porosity characteristic of articles made by powder metallurgical techniques. In many cases this porosity is a desired attribute as, for example, when the corrosion resistant alloy body is to be used as a filter or other permeable element. However, if it is desired that the metal alloy have a dense, nonporous structure the alloy body can be compacted by cold rolling or by hot rolling in a controlled atmosphere to obtain the desired nonporous structure.

The corrosion resistant copper-nickel alloy of my invention is particularly useful as a cladding material for sheet steel or other metal substrates. In this embodiment of my invention the mixture of metal powder is spread in a layer of the desired thickness on the surface of the sheet metal substrate advantageously in accordance with the general procedure shown in U.S. Patent 2,815,567. In a modification of this procedure the mixture of metal powder is admixed with a heat fugitive organic binder as previously described before being spread onto the surface of the metal substrate. The sheet metal substrate and the layer of metal powder mixture thereon are advantageously rolled to compact the metal powder, and then are heated to simultaneously form the nonhomogeneous alloy of my invention and firmly bond the alloy layer to the underlying metal substrate. The metal substrate with the adhering layer of corrosion resistant alloy thereon is advantageously rolled to compact the porous layer of alloy particles to obtain a nonporous corrosion resistant layer on the metal substrate. In a further modification of the practice of my invention, the mixture of metal powder can be applied to one or both sides of the metal substrate as illustrated in U.S. Patent 2,979,400. Moreover, as described in the latter patent, the metal substrate can itselfbe made of a porous corrosion resistant material such as a cupro-nickel wire mesh screen whereby the resulting nonhomogeneous alloy structure is useful as a filter element or medium in corrosive atmospheres.

The following examples are illustrative but not limitative of the practice of my invention.

EXAMPLE I Thirty-five parts by Weight of electrolytic copper powder having a particle size of minus 325 mesh (Tyler Standard) is thoroughly mixed with parts by weight of electrolytic nickel powder having a particle size such that substantially all of the powder was smaller than 150 mesh and larger than 325 mesh (Tyler Standard). The mixture of metal powder is placed in a cylindrical die cavity and compacted with a force of 10,000 psi. to obtain a powder metal compact having a diameter of one inch and a thickness of /8 inch. The metal powder compact is then heated in a reducing atmosphere containing about 10 percent hydrogen gas at a temperature of 1100 C. for a period of one minute. The resulting metal disk comprises a coherent metal body composed of mutually adhering particles of a nonhomogeneous copper-nickel alloy, the surface layer of each particle containing about percent by weight copper and the center of each particle containing about percent by weight nickel. The metal alloy body has a porous structure and contains no free (that is, unalloyed) copper.

EXAMPLE II The porous metal alloy disk produced as described in Example I is rolled to reduce the thickness of the disk by about 50 percent and to obtain a nonporous metal alloy body having an apparent density approaching the theoretical density of copper-nickel alloys containing 35 percent by weight copper and 65 percent by weight nickel.

EXAMPLE III Thirty parts by weight of electrolytic copper powder having a particle size of minus 325 mesh (Tyler Standard) is thoroughly mixed with 70 parts by weight of electrolytic nickel powder substantially all of which has a particle size of smaller than mesh and larger than 325 mesh (Tyler Standard). One part by Weight of polyisobutylene (sold under the trade name of Vistanex L300) is dissolved in 23 parts by weight naphtha. One hundred and twenty parts by weight of the mixture of metal powder is thoroughly mixed with 24 parts by weight of the resinous solution to obtain a heavy, spreadable slurry of the metal powder in the resinous solution. The slurry is spread onto a moving belt in a layer having a thickness of from 0.01 to 0.02 inch, and then is heated in a vaporization furnace at a temperature of about 400 C. to volatilize and drive off the naphtha to obtain a dried metal and plastic strip. The metal and plastic strip is a self-supporting structure, and it is heated at a temperature of 1100 C. for a time sufiicient to drive off the polyisobutylene constituent of the strip and to cause the copper constituent thereof to melt and alloy with the unmelted nickel particles.

EXAMPLE IV The porous metal strip obtained by the procedure described in Example III is rolled to reduce the thickness of the strip by about 50 percent and thereby obtain a substantially nonporous nonhomogeneous copper-nickel alloy strip having an apparent density approaching the theoretical density of a homogeneous copper-nickel alloy of the same composition.

From the foregoing description of my new nonhomogeneous copper-nickel alloy it will be seen that I have made an important contribution to the art to which my invention relates.

*What is claimed is: a a 4 4.

1. In the method of making corrosion resistant nonhomogeneous copper-nickel alloys in which a mixture of finely divided cop-per powder and finely divided nickel powder is heated in a reducing atmosphere to a temperature above the melting point of copper and below the melting point of nickel for a time sufficient to cause the copper to alloy with the nickel powder and to sinter the metal powder particles together, the improvement which comprises:

forming a mixture containing from about percent to 50 percent by weight of finely divided copper powder and from about 50 percent to 80 percent by weight of nickel powder, the average particle size of the copper powder being less than the average particle size of the nickel powder,

forming the mixture of metal powder into an article of the desired shape, and

heating the shaped mixture of powder for a time sufficient to cause substantially all of said copper to alloy with the unmelted particles of nickel powder and to form a nonhomogeneous copper-nickel alloy the individual particles of which comprise not more than about 70 percent by weight copper at the surface thereof and about 100 percent by weight nickel at the center thereof.

2. The method according to claim 1 in which the particle size of substantially all of the copper powder is less than about 325 mesh (Tyler Standard) and the particle size of substantially all of the nickel powder is less than about 65 mesh, the preponderance of particles of the copper powder having a substantially smaller particle size than the particles of nickel powder.

3. The method according to claim 1 in which the initial mixture of copper and nickel powders contains about percent by weight copper powder and about 65 percent by weight nickel powder.

4. The method according to claim 1 in which the shaped mixture of metal powder is heated in a reducing atmosphere containing from about 6 to 100 percent hydrogen.

5. The method according to claim 1 in which the shaped mixture of metal powder is heated at a temperature of about 1100 C. for a period of about one minute.

6. The method according to claim 1 in which the mixture of metal powders is compacted to form a metal powder article of the desired shape prior to heating to form the nonhomogeneous alloy.

7. The method according to claim 1 in which the nonhomogeneous metal alloy shape is subjected to a rolling operation following the heating step whereby a dense substantially nonporous metal structure is obtained.

8. The method according to claim 1 in which the mixture of metal powder is spread in a relatively thin layer on the surface of a metal substrate, and in which the nonhomogeneous particles of nickel-copper alloy obtained as a result of the heating step adhere to each other and to the underlying metal substrate to form a coherent layer of corrosion resistant alloy on said surface thereof.

9. The method according to claim 8 in which the metal substrate with the adhering coherent layer of corrosion resistant alloy thereon is subjected to a rolling operation following the heating step to obtain a dense substantially nonporous layer of corrosion resistant alloy on said metal substrate.

10. The method according to claim 1 in which the mixture of nickel and copper powders is admixed with a solution of a fugitive resin in a volatile solvent, in which the mixture of metal powders and resinous solution is spread in a relatively thin sheet, in which said sheet is heated to a temperature above the boiling point of the volatile solvent and below that of the fugitive resin to evaporate the volatile solvent and thereby obtain a self-supporting sheet of metal powder bonded together by the fugitive resin, and in which said self-supporting sheet of metal powder and fugitive resin is heated to a temperature above the melting point of copper and below the melting point of nickel to drive off the resin and to form a coherent sheet of said non-homogeneous corrosion resistant alloy.

11. The method according to claim 10 in which the mixture of metal powder and resinous solution is spread on the surface of a metal substrate, and in which the particles of nonhomogeneous copper-nickel alloy obtained as a result of the second heating step adhere to each other and to the underlying metal substrate to form a coherent layer of corrosion resistant alloy thereon.

12. The method according to claim 11 in which the metal substrate With the adhering coating of corrosion resistant alloy thereon is subjected to a rolling operation following the second heating step to obtain a dense substantially nonporous layer of corrosion resistant alloy on said metal substrate.

References Cited UNITED STATES PATENTS 2,241,094 5/ 1941 Marvin 208 2,290,338 7/1942 Koehring 75208 X 2,819,962 1/1958 Salauze 7S208 2,855,296 10/1958 Koehring 75208 3,086,860 4/1963 Montaud 75-222 3,197,847 8/ 1965 Kerstetter 75--21l X 3,323,879 6/1967 Kerstetter 75-222 X 3,335,002 8/1967 Clarke 75214 X FOREIGN PATENTS 586,895 4/ 1947 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

A. J. STEINER, Assistant Examiner.

US. Cl. X.R. 

